When I have a large quilt to square up I clear furniture and spread it out on the floor. I get all my quilting squares and rulers, my long tape measures and also long squares/rulers from my husbands workshop. I place the squares on the corners of the quilt, with rulers butting up to these, also placing some weights on the rulers so they do not move.
Next I measure with my tapes (horizontally, diagonally and vertically). I shuffle and nudge until things seem pretty good. I then use a pigma pen to draw the line where the quilt will finish.
In the absence of squares I use large sheets of card (cardstock) or my cutting mats to get the right angles, and then use rulers in-between. It is a case of having a look around your home to see what you have that will do the job. Anything straight and with true right angles will work!